1. Field of the Invention
The invention pertains to the field of liquid level sensing. More particularly, the invention pertains to an apparatus and method for the detection of low liquid levels in containers to produce an audible and/or visual alert indicative of a low liquid level condition in such containers.
2. Description of Related Art
Chemical processes are often fed automatically using pumps which are activated based on some property of the process such as time, production workload, or a monitoring device which could be either gravimetric, colorimetric, pH, or ORP-based for example. As the process requires it, chemicals are dispensed from a container such as a standard 55-gallon drum. If the contents of the drum are depleted, the process may be harmed or defective product may be produced. To avoid this, chemical process operators must be always on alert. One possible way around this taxing burden is to have a dedicated permanent “day tank” which is equipped with a permanent alarm, but this requires the effort of having to pump possibly toxic or dangerous chemicals into the “day tank”, and it takes up space. It is much more convenient to feed chemicals directly out of the containers they are shipped in. Therefore, the need exists to automatically alert the process operators when such containers are empty.
One class of inventions represented by CHAMBERLAIN et al. U.S. Pat. No. 6,727,822 comprises a float that responds to changing liquid level by closing an electrical circuit. In industrial manufacturing environments, this reliance on 120 volt power is unreliable, or in wet environments, possibly dangerous to the operator.
Alternatively, many level detection devices to date have relied on conductivity probes to detect low liquid level as in:                DICKIE et al. U.S. Pat. No. 6,337,632,        HINSHAW et al. U.S. Pat. No. 4,279,078        SHEFSKY U.S. Pat. No. 5,065,139        HOHMNAN U.S. Pat. No. 6,873,263        
Conductivity probes must be made from conductive metal and therefore the potential exists for chemicals contained in the drum to attack or foul the probes—leading to failure. Also, while these probes may be satisfactory for conductive liquids, they are useless for non-conductive liquids. Conductive probes have the further disadvantage that they must consume power to do frequent checking of the level. In a battery-operated device where power conservation is vital, this is a substantial disadvantage. Moreover, conductivity probes are prone to failure if enough condensation or splashed liquid is present on the probes to provide a conductive path. In this failure mode, an alarm would not be given when a drum is actually empty.
A different approach that avoids this parasitic power consumption is that of SHUKLA et al. U.S. Pat. No. 5,767,775 which describes a mercury switch and transmitter entombed in a Teflon™ cone, however this device is not easily retrieved from the interior of portable containers such as a chemical drums (if it could be made to fit in the bung hole in the first place) which makes it unsuitable for constantly transferring to new incoming drums. In addition, there is no confirmation that the device is even in the drum that liquid is being drawn from, and no way to check that it is working properly in closed containers like drums. Furthermore, there is no way to adapt this device to sense high liquid levels. Checking the function of the device is difficult or impossible. Other float type inventions are that of GALLAGHER U.S. Pat. No. 5,999,101 which describes a float which activates an alarm outside the container, however it consists of an unwieldy and cumbersome apparatus inside the container that is suitable only for permanent mounting in tanks—not to be moved from drum-to-drum. Doing a functional check of the unit is impossible without reaching down into the tank and picking up the float.
FREILL et al. U.S. Pat. No. 6,414,598 describes a battery-operated and float device suited for shallow pans. It could conceivably be adapted for use with chemical drums if the float could be made to fit inside the bung hole of a drum, and conceivably it could be made to operate in a low level mode, (instead of high level) and possibly the electronics could be even encased in a chemical-proof housing, but it has no flashing light—which is essential in a factory environment which might have dozens of alarms all within earshot. Further, the float height is adjustable, and this degree of operator discretion is unwelcome in a factory where standardization is the goal. Finally, the device with its exposed string is simply nowhere near robust enough for industrial use.
Also lacking a flashing light, is the invention of ESPOSITO U.S. Pat. No. 6,480,113. The floating member makes a direct electrical connection in the alarm circuit. This exposed electrical contact would not be nearly reliable enough in a factory setting with corrosive fumes; the design of the float sliding within the electrical housing precludes sealing against these corrosive fumes. Further, the level-sensing member is not nearly long enough to sense liquid at the bottom of a drum.
None of the prior art provides any way to ensure that the suction tube is located in the drum with the alarm, and no way to ensure that the suction tube is at the correct depth. (Which must be below the level of the float, or the process may be starved for chemicals for some time before an alarm is given) Finally, none of the prior art provides a convenient means of monitoring the flow of liquid from a level-alarming device.